Heretofore, there is known an apparatus for detecting the liquid level by using X-rays for measuring the liquid volume of a sealed container, as shown in Japanese Patent Laid-open Publications No. H05-322630 A and 2002-357472 A
In the above conventional method, since X-rays are irradiated in a horizontal direction with respect to the liquid surface, it can be detected only in the case of a liquid having a constant liquid level in the entire container, and no consideration has been given to measuring the liquid volume of a minute amount of locally present liquid.